Nuclear reactors have fuel assemblies consisting of an array of individual fuel rods that contain uranium pellets. The uranium pellets are produced by pressing and sintering ceramic uranium dioxide, UO.sub.2, powder. The strength and integrity of the pellets depends on the particular process used to produce the uranium dioxide powder. Ultimately, high performance in nuclear reactors depends on the microstructure of the individual particles in the manufactured uranium dioxide powder. Thus, the process selected for manufacturing uranium dioxide powder is important.
A process that produces uranium dioxide powder with exceptional microstructure characteristics for nuclear fuel pellets is the Ammonium Diuranate process. The Ammonium Diuranate process begins with hydrolysis of uranium hexafluoride gas. An aqueous mixture of uranyl fluoride, UO.sub.2 F.sub.2, and hydrofluoric acid, HF, called hydrolyzed uranium fluoride, is formed. Subsequently, the hydrolyzed uranium fluoride is contacted with aqueous ammonia, NH.sub.4 OH, to produce an Ammonium Diuranate precipitate.
The precipitation reaction occurs in one or more stages and in the presence of excess ammonium hydroxide to promote optimal Ammonium Diuranate particle nucleation and growth. The slurry produced in the precipitation reaction is filtered or centrifuged before it is transferred to a kiln for reduction and defluorination. In the kiln, the Ammonium Diuranate precipitate is converted first to uranium oxides such as U.sub.3 O.sub.8, then reduced to uranium dioxide. Ultimately, traces of fluoride are removed from the uranium dioxide powder by stripping with a small amount of steam.
While the ceramic properties of the uranium dioxide powder produced from the Ammonium Diuranate process are excellent, the conventional process produces an undesirable liquid waste stream. The liquid waste stream consists of virtually all of the water used in the various reactions above, traces of soluble and particulate uranium, excess ammonium hydroxide, and ammonium fluoride. In fact, nearly all of the fluoride originally present in the hydrolysis of uranium hexafluoride, leaves the process as ammonium fluoride.
This relatively large flow of mixed chemicals is expensive and difficult to treat. Processing methods to treat the liquids include ultrafiltration, centrifugation, ion-exchange, solvent extraction, lime precipitation, ammonia distillation, and press filtration, to mention a few.
Ammonium Diuranate treatment processes include steps to recycle uranium, other steps to recycle ammonia, and yet more steps to dispose of the fluorides by neutralizing with lime so as to produce calcium fluoride. The calcium fluoride must be sent to a landfill or disposed of by other means. Disposal of such salts is proving to be increasingly expensive because of the high cost of capital and the stringent environmental regulations.
Thus, there is a need to have an effective process for treating fluoride-containing aqueous streams. There is also a need to have a process that significantly reduces the amount of waste disposal. Finally, there is a need for a process to recover the fluoride in the waste stream and convert it into a usable and salable product.